1. Field of the Invention
This invention relates to an exposure apparatus for transferring patterns, and in particular to an illuminating optical system using a laser source.
2. Related Background Art
An exposure apparatus having as a light source an excimer laser emitting deep ultra-violet rays has drawn attention as a transfer apparatus for fine LSI patterns smaller than 0.5 .mu.m rule. Particularly, an apparatus of the type which uses a band-narrowed laser whose oscillation wavelength width is narrowed, and which reduction-exposes the pattern of a negative such as a reticle or a mask onto a photosensitive substrate such as a wafer by a projection lens using only quartz as an optical material is expected to be put into practical use soon.
An excimer laser source has an asymmetrical light flux density distribution and therefore causes illumination distribution irregularity in a negative illuminated. U.S. Pat. No. 4,769,750 proposes an illuminating optical system which solves this problem.
Further, a beam emitted from an excimer laser source has the tendency that spatial coherence is high. For example, when an injection locking laser of the master and slave type is used, there is provided an illuminating light which is small in the number of oscillation modes and high in coherency, and unnecessary interference fringes called speckles superposed on the transferred pattern tend to appear on a wafer.
Also in a narrow-band laser in which an etalon (what is provided with two parallel flat plates of quartz disposed at a predetermined interval) or a spectral grating is inserted into a laser resonator to keep the beam diameter large and, in which the number of oscillation modes is not increased, there has been the problem that spatial coherence is higher than before the band is narrowed and particularly, for a direction in which the angle of divergence of the beam is small, spatial coherence is high and unnecessary interference fringes tend to appear.
U.S. Pat. No. 4,619,508 teaches several methods for forming an incoherent light beam from a coherent light beam.